Even though a lot has been said about room acoustics, it is still a topic people have a lot of questions about. In this article I want to give a brief introduction to room acoustics and how to identify and prevent possible problems in your listening room. All sound coming from your speaker is reflected and distorted at the walls of your listening room. These distorted reflections combined with the direct sound coming from the speakers create the sound field you are listening to. Depending on the shape of the room and on the surfaces of the walls, the sound field will change accordingly. Hence you won’t find two rooms which acoustically behave exactly the same and room treatment has to be well adapted for each individual room.
The human ear is very well trained in hearing room size and the structure of its walls. You don’t believe me? Just try it! Go to your bathroom, clap your hands and listen to the room. Then do the same in your living room. (If it sounds the same you should consider getting some furniture and stop spending all your money on gear). The difference should be obvious but what exactly do we perceive differently in these two rooms?
I’m guessing your living room sounds way more dull or muffled than your bathroom. This is due to different absorption coefficients of materials [The absorption coefficient alpha describes how much a certain frequency band is absorbed by the material. Alpha = 1 means all sound energy is absorbed, Alpha = 0 means all sound energy is reflected]. Check this list of absorption coefficients for different materials. If you look up the values for typical furniture in living rooms, you can see that most of it is very absorbent in higher frequency bands. That’s why the living room sounds muffled compared to the bathroom. In addition, reflected sound travels longer and therefore arrives later at the listening position as the direct sound of the speaker. The reflections arriving at your ear after one or two reflections are known as early reflections.
It’s easy to identify the early reflections in the Impulse response of your room. If you haven’t heard of impulse responses yet, here is a brief explanation: A Impulse response describes the reaction of a dynamic system to some external change. In this case the dynamic system is our listening room and the external change is an Impulse we generate in the room. The Impulse has to be very short and must have a flat spectrum. Short transient noises like gunshots or balloon burst can offer close enough approximations for practical measurements. But please don’t start shooting in your studio now!
When we look at the Impulse response of a room we can identify a high peak in the beginning and several peaks, lower in amplitude, some milliseconds later. The highest peak in the beginning is the direct sound coming from the loudspeaker. The peaks coming after the direct sound are the early reflections from reflecting surfaces like your floor, a side wall or your mixing desk. The tail after the early reflections is called reverb. It’s basically a lot of reflections arriving at the microphone or your ears more or less at the same time. That’s why you cannot distinguish separate peaks anymore.
Reverberation time (RT)
Due to a large amount of sound absorbing materials and scattering objects in a typical domestic listening room the reverberation time is very short compared to performance spaces like concert halls (0.2 to 0.6 seconds compared to 1.5 to 5 seconds in large performance spaces). Therefore, what we perceive in small rooms is very much dominated by the directional characteristics of the loudspeakers and the acoustic behavior of the room boundaries at the locations of the strong early reflection. A reverberation time measurement says nothing about this! This doesn’t mean that it’s a technically incorrect measurement, it’s just not useful for telling us how reproduced music in small rooms will sound like. Nonetheless highly reflective or reverbant rooms aren’t considered as good listening rooms. For that reason, a RT measurement can help identifying if you are going in the right direction when treating your room acoustically.
Loudspeakers and rooms interact differently below and above a transition region around 200 to 300 Hz. The transition frequency between these two sound fields is called Schroeder frequency. In larger rooms it can be easily calculated with a formula depending on the reverberation time and the room volume. This calculation assumes meaningful reverberation times which are not given in small rooms and hence almost always leads to errors in calculation. Nevertheless, the transition region exists and it rises as the room shrinks.
High frequency behavior
For mid and high frequencies, where wavelengths are short compared to the room dimensions the concept of ray acoustics is appropriate. As mentioned above we hear a combination of direct sound and reflections. The frequency response depends a lot on the off-axis response of the loudspeaker. If the speaker doesn’t have a constant or smoothly changing directivity, this yields to ripples in the frequency response of the room. When we are designing speakers at EVE Audio we optimize them until their on-axis as well as their off-axis behavior is optimal to get a perfect listening experience in almost all rooms.
The aim of acoustic treatment is to compensate irregularities of the speaker directivity with absorption material at the right positions and with appropriate sound absorbing characteristics. Another important thing to consider is the so-called precedence effect. This psychoacoustic phenomenon says: When two sounds reach your ear from different directions, separated by a short time delay, you’ll localize the sound source at the position where the first wave front came from. This effect works best when the spectra of the two sounds are similar. If the spectrum of the reflection is different from that of the direct sound, its very likely you hear separate spatial events. That’s something we definitely want to avoid! So if you apply absorbent material to dampen early reflections the absorbent material should be uniformly effective above the transition frequency.
Low frequency behavior
When the dimensions of the room become significant compared to the wavelengths the behavior of the room is better described in terms of sound waves. At these lower frequencies room modes (or room resonances) dominate the behavior of small rooms. The frequencies of modes and their distribution can be altered by adjusting the room dimensions and the room proportions. Many people claim there is an optimum for room dimensions to have a perfect listening experience but besides the fact that you normally cannot change the size and shape of your room there is another problem with this concept.
The concept assumes all modes are excited simultaneously and the listener hears all of them equally. That’s just possible if you position a loudspeaker in one room corner and listen back to it in the opposite one. Any deviation from these positions yield either to not all of the modes are equally excited or not equally audible. The concept is not wrong, it’s just not applicable for sound reproduction. But there are of course ways to deal with resonances.
To get rid of annoying resonances at one listening position in a stereo setup most of the times using an equalizer is enough to adjust the bass. But if you want to deliver good bass to several listeners it gets way more complicated. You have to physically attenuate the energy of resonances with bass absorbers. Unfortunately, these absorbers have to be very large to efficiently attenuate low frequencies. Most of the times they are installed in room corners or realized as a wall construction. Another way to reduce the effects of one particularly irritating mode is to locate the subwoofer near its pressure minimum. Doing this will cancel out the chosen resonance.
In summary, the sound field of loudspeakers in small rooms can be divided into two regions. Below 300 Hz and above it. There are different ways to deal with the problems occurring in these two different sound fields and unfortunately there is not one magical absorber that makes your room instantly perfect. If you have carefully designed loudspeakers with a smoothly changing directivity and a flat on-axis frequency response its most likely that you don’t need much acoustical treatment to enjoy your listening experience. At least for higher frequencies. The low end can still make problems, but carefully placing the speakers and/or subwoofer most of the time helps already a lot to get a good bass response.
If you want more in-depth insight into room acoustics and loudspeakers, I highly recommend Floyd E. Toole’s book ‘Sound Reproduction – Loudspeakers and rooms. It’s a good read and clears up with a lot of misunderstandings about room acoustics. I also used it as main source for this post!
Written by Lucas Weidinger
(R&D Engineer at EVE Audio GmbH. He studied electrical engineering, computer science and acoustics at the technical university of Berlin)
Picture Source: RELATIVE DISTANCE PERCEPTION OF SOUND SOURCES IN CRITICAL LISTENING ENVIRONMENT VIA BINAURAL REPRODUCTION – Scientific Figure on ResearchGate. [accessed 3 Oct, 2019]